Blood Group Antigen Types and Prevalence
Blood groups, or types, are defined by polymorphisms in the expression of immunogenic molecules in blood cells, plasma, and body secretions such as milk and saliva. A variety of these can be found across species, including in humans and great apes. There are several different categories of blood group antigens described in humans that have great clinical importance. The most commonly described of these, because of their high polymorphism in human populations and clinical relevance, are the ABO blood groups and the Rhesus (Rh) factor.
The human ABO blood group system is very significant in transfusion and transplantation, and with a few exceptions remains polymorphic in most human populations. These antigens are oligosaccharides syntheiszed by glycosyltransferase enzymes, which create a pattern of sugars present on the outer membrane of cells or secreted glycoprotiens. In type O blood, a carbohydrate sequence called the H antigen is present. Type A and B individuals modify the H antigen by adding an additional monosaccharide to produce corresponding A and B antigens. Individuals produce antibodies against the antigens not present in their own blood. This process is presumed to occur through early exposure to bacteria with similar antigens. The presence of ABO polymorphism is highly variable across all primates. Chimpanzees have been found thus far to have primarily type A blood, with type O less commonly. Gorillas appear to be exclusively type B. Orangutans express all three blood types.
Unlike the ABO blood group, the Rh blood group antigens are proteins. Rh type is important in humans because of its role in hemolytic disease of the newborn, in which the antibodies of an Rh- woman (lacking the D antigen protein of the Rh blood group) target the D antigen on the red blood cells of an Rh+ fetus. Rh polymorphisms also exist in chimpanzees, where they were originally described in the R-C-E-F blood group system. Chimpanzees share some of the variants of Rh with humans, but the two species have additional variants that they do not share. Pre-immunization of chimpanzees with D antigen can cause hemolytic disease of the newborn in subsequent pregnancies; however, this has not been documented to occur naturally.
Other, less commonly studied blood groups also have correlates in chimpanzees: the MNS blood group system (V-A-B-D in chimpanzees), and the Ii blood group (not polymorphic in chimpanzees).
Genetic analyses of the ABO blood group system have suggested that the same antigens have emerged multiple times throughout primate evolution, and that polymorphism is maintained by balancing selection within populations. This may indicate that the maintenance of diversity within various blood group antigens is important for population avoidance of pathogens. Large numbers of viral and bacterial pathogens have been shown to bind ABO types differentially. However, the pressures underlying the various blood group antigens are not yet completely understood. Regardless, despite their clinical significance, the blood group antigens are not useful in understanding human uniqueness.
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